1. Field of the Invention
The present invention relates to a method of and an apparatus for producing potable water and special salt from salt water (seawater or brackish water). The special salt means salt containing salts in the salt water.
2. Description of the Prior Art
In order to solve chronic shortage of water in isolated islands, a seawater desalting method using a reverse osmosis (RO) membrane has been embodied in recent years. In the seawater desalting method, permeated water obtained by the reverse osmosis membrane is utilized as potable water, and concentrated salt water is discharged into a sea area.
On the other hand, electrodialysis (ED) has been used in order to produce common salt. In the electrodialysis, seawater is concentrated using an ion exchange membrane, and moisture of concentrated salt water obtained is evaporated, to deposit sodium chloride (NaCl).
According to the above-mentioned seawater desalting method using the reverse osmosis membrane, the concentrated salt water having a higher salt concentration than that of the seawater is discharged into the sea area, whereby it is feared that weeds and fishes such as the young of fishes and shellfishes are adversely affected. The seawater is sterilized using chlorine in pretreatment using the reverse osmosis membrane, whereby there is also a possibility of phytotoxicity. Further, the recovery of the reverse osmosis membrane is approximately 30 to 40%. Therefore, 60 to 70% of the seawater pretreated at high cost is discharged as it is into the sea area, whereby the utilization factor of the seawater is low, and the cost of equipment and the operating cost are increased.
On the other hand, salt produced by the electrodialysis is composed of only NaCl, whereby it cannot be said that it tastes good. Therefore, salt obtained by mixing an additive such as bittern with common salt is preferable.